1. Field of the Invention
The present disclosure relates to an element isolation of a semiconductor device, and particularly, to an element isolation structure of a semiconductor, capable of forming an element isolation structure by using a deep trench and a shallow trench without any defect, and a method for forming the same.
2. Background of the Invention
Recently, element isolations for high voltage devices, among semiconductor devices, are divided into a junction isolation method and a deep trench isolation method. In recent time, the element isolation is achieved by the deep trench isolation for size reduction and improvement of isolation characteristics. For devices using an isolation technique below 0.25 μm CMOS design rule, trenches of a high voltage device and a low voltage device have different trench depths, so a dual-depth trench having a deep trench and a shallow trench should be formed.
A method of forming an element isolation structure of a semiconductor device, in which a shallow trench is formed after forming a deep trench is formed, will be described with reference to FIGS. 1A to 1I.
FIGS. 1A to 1I are sectional views showing sequential processes of a method for forming an element isolation structure of a semiconductor device according to the related art.
Referring to FIG. 1A, a semiconductor substrate 11 having an inactive region and an active region is sequentially deposited with a pad oxide 13 and a pad nitride 15, and then a first photosensitive film 17 is coated on the pad nitride 15.
Referring to FIG. 1B, the first photosensitive film 17 is patterned through a photolithography process using a mask (not shown) via an exposure and development process, thereby forming first photosensitive film patterns 17a. 
Referring to FIG. 1C, the pad nitride 15, the pad oxide 13 and the semiconductor substrate 11 are sequentially etched out by using the first photosensitive film patterns 17a as masks, thereby forming a deep trench 21 into the semiconductor substrate 11.
Referring to FIG. 1D, after removing the first photosensitive film patterns 17a, a second photosensitive film 23 is coated on the pad nitride 15 including the deep trench 21, thereby filling the deep trench 21.
Referring to FIG. 1E, the second photosensitive film 23 is exposed by irradiating ultraviolet rays 27 on the second photosensitive film 23 through a photolithography process using masks 25. Here, upon exposure of the second photosensitive film 23, the exposed portion of the second photosensitive film 23 is physically transformed. However, the depth of the deep trench 21 may interrupt the exposure of the entire second photosensitive film 23 present within the deep trench 21, and accordingly a bottom portion 23a of the second photosensitive film 23 remains unexposed.
Referring to FIG. 1F, the exposed portion 23b (see FIG. 1E) of the second photosensitive film 23 is melted by a development solution but the unexposed portion 23a of the second photosensitive film 23 is not melted thereby. The exposed portion 23b of the second photosensitive film 23 is melted out through the development process, but the unexposed portion 23b of the second photosensitive film 23 on the bottom of the deep trench 21 still remains unexposed.
Referring to FIG. 1G, the pad nitride 15, the pad oxide 13 and the semiconductor substrate 11 are sequentially etched out by using the remaining second photosensitive film 23 without being removed as a barrier layer, thereby forming a shallow trench 31 in the semiconductor substrate 11. Here, during the etching process, the remaining portion 23a of the second photosensitive film 23 on the bottom of the deep trench 21 may interrupt the etching, thereby causing the peripheral portion of the remaining second photosensitive film portion 23a to be etched out, resulting in the generation of notches 33.
Afterwards, the second photosensitive film 23 and the remaining photosensitive film portion 23a on the bottom of the deep trench 21 are removed.
Referring to FIG. 1H, sidewalls of the deep trench 21 and the shallow trench 31 are oxidized so as to form an oxide 35 on the sidewalls. Here, growing of the oxide 35 allows overcoming of a defect on the semiconductor substrate 11 present on the trench sidewalls.
Still referring to FIG. 1H, a filling oxide 37 is formed on an entire surface of the substrate including the deep trench 21 and the shallow trench 31, thereby filling both the deep and shallow trenches 21 and 31.
Referring to FIG. 1I, the filling oxide 37 present outside the deep and shallow trenches 21 and 31 is planarized through a chemical mechanical polishing (CMP) process, thereby forming a filling oxide pattern 37a within the deep and shallow trenches 21 and 31. Here, the filling oxide pattern 37a constructs an element isolation structure 10, which includes a deep trench region 10b and a shallow trench region 10a. 
Although not shown, the pad nitride 15 may selectively be removed so as to complete a process of forming the element isolation structure 10 of the semiconductor device.
However, the method of forming the element isolation structure of the semiconductor device according to the related art has several problems as follows.
According to the related art method of forming the element isolation structure, when the deep trench is first formed, it may be possible to remove an undercut portion during a shallow trench etching process. However, a defective coating may be caused during a photolithography process for forming the shallow trench, resulting in giving rise to a problem during the pattern forming process. That is, upon exposure of the photosensitive film, the exposed portion of the photosensitive film is physically transformed but the depth of the deep trench may interrupt the exposure of the entire photosensitive film within the deep trench. Consequently, the bottom portion of the photosensitive film may remain unexposed. As a result, the exposed portion of the photosensitive film is melted out by a development solution through a development process but the unexposed portion of the photosensitive film present on the bottom of the deep trench remains unexposed.
The remaining portion of the photosensitive film may interrupt etching during the following etching process of forming the shallow trench, which causes the peripheral portion of the remaining photosensitive film portion to be etched out, thereby generating the notches.
Also, according to the related art method of forming the element isolation structure, when coating the photosensitive film for forming the shallow trench after forming the deep trench, since the photosensitive film is coated by a spin coating, a sufficient gap-filling is not made. Accordingly, a void is generated, thereby causing a defective coating. That is, since the deep trench is extremely deep in depth, the defective coating such as the void is caused, whereby a material such as the photosensitive film (PR) is not evenly filled up to the top of the entire substrate.